The invention relates to an optical device for measuring the thickness of a first medium that is at least partially transparent for an incident beam and covers a second medium, the device comprising means for generating the incident light beam so as to form a beam reflected by the surface of the first medium and a beam scattered by the surface of the second medium, detection means for detecting the reflected beam and the scattered beam, processing means, connected to the detection means, comprising means for measuring the gap between the reflected beam and the scattered beam, and means for determining the thickness of the first medium according to the measured gap.